Current transformers (CT) are instrument transformers that are used to supply a reduced value of current to measurement instruments (e.g. meters). CT provide isolation from a high voltage primary circuit, permit grounding of a secondary circuit, and step-down the magnitude of the measured current to a value that can be safely handled by the measurement instrument.
The most common CT secondary full-load current is 5 A, matching the typical standard 5 A full-scale current rating of traditional power metering equipment that are CT driven.
A typical design for a CT consists of a length of wire wrapped many times around a ferrous core (e.g. a steel ring) that is passed over the circuit being measured. The CT primary circuit consists of a single turn of conductor (i.e. the conductor being measured) and the secondary circuit consists of multiple turns.
Each CT has a current-ratio expressed as a ratio of the maximum rated primary current to the maximum rated secondary current. For example, a 300:5 CT will produce 5 A of secondary current when 300 A flows through the primary. As the primary current changes the secondary current varies accordingly. When 150 A flow through the 300 A-rated primary, the secondary current is 2.5 A. When the current through the primary exceeds the rated maximum, the current in the secondary will increase but will not, in general, be in proportion in accordance with the current ratio.
CT are defined by Accuracy Classes depending on their intended application. For applications such as, for example, metering for revenue billing, metering accuracy CT in accordance with, for example, American National Standard Institute (ANSI) C57.13 or International Electrotechnical Commission (IEC) 44-1 are used.
With the rising cost of energy and the growing concern over environmental impact, there is a desire to increase efforts directed toward reducing energy consumption. In the case of alternating current (AC) electrical power, consumption has traditionally been measured on an aggregated basis using, for example, one meter per home or per building. Conservation efforts are more effective when consumption can be measured on a more granular basis such as, for example, on a per household (e.g. in the case of a multi-residence building) or per circuit basis. Mechanisms directed to more granular measure are sometimes referred to as sub-metering.
In sub-metering applications, for example, it is often desirable to introduce metering points for multiple clients in an arrangement known as multi-customer meter systems (MCMS). The MCMS can include multiple metering points within a single enclosure and/or metering points that are distributed through a building or complex.
FIG. 1 is a schematic representation of a typical current transformer based metering arrangement. In a typical metering application a CT 110 is applied to each circuit 950, between an AC source 910 and a load 920, for which individual consumption measurements are desired. Traditional CT 110 are used to both sense current flowing in the circuit 950 and to step down the measured current as direct measurement of large currents is generally not desirable. The stepped down current is sensed across a precision resistor (not illustrated) within a measuring instrument 940 wherein the voltage drop (i.e. potential difference) across the resistor is directly proportional with the measured current. Within the measuring instrument 940 the measured voltage drop can be digitized and input to a digital signal processor (DSP) that can apply a ratio multiplier, based on the characteristics of the CT 110 to generate a current measurement. The measured current can then be multiplied by the voltage drop measured across load 920 to derive a power consumption measurement.
In a traditional metering application a measured current of 100 Amperes (A) or more is stepped down to a measurement current of not more than a threshold level such as, for example, up to 5 A, 10 A or 20 A by the CT. These CT are referred to as Ampere output CT. The CT is typically a toroidal transformer with the circuit to be measured passing through an aperture in the toroidal core of the CT. The current in the circuit being measured is referred to as the primary current. A winding around the toroidal core (a.k.a. the secondary circuit) generates an output current (a.k.a. a secondary current) that is proportional to the primary current. The proportion (i.e. ratio) of the primary current to the secondary current is a function of the ratio of the number of turns in the transformer windings that is referred to as the turns-ratio or the current-ratio.
The Ampere output CT have a relatively lower burden rating (i.e. the load resistance that the secondary current can drive while remaining within the specified accuracy of the CT) such as, for example, 0.2 ohms for a 5 A CT. The low burden rating requires that the measurement instrument 940 be located relatively close to the CT 110 (e.g. within approximately 20 feet). Although the use of larger gauge (e.g. larger than 16 American wire gauge (AWG)) wire to connect the CT 110 to the measuring instrument 940 permits the distance to be increased, this solution is costly and increasingly impractical as the wire gauge size and/or the distance increases. The secondary current (e.g. up to 5 A) of the Ampere output CT also results in relatively higher heat dissipation in the measuring instrument.
Recent measuring instruments are designed to operate with lower input currents typically on the order of up to 40 milliamperes (mA) or 80 mA and therefore are typically used with CT that have correspondingly lower secondary currents. These CT are referred to as mA output CT.
The mA output CT have a relatively higher burden rating such as, for example, 15 ohms for a 80 mA CT, that permits a relatively greater distance between the CT 110 and the measurement instrument 940. However, mA output CT are not readily available for primary currents greater than 200 A. Therefore mA output CT typically cannot be used in high current (e.g. greater than 200 A) applications.
In addition, in most jurisdictions electrical current supply and instrumentation (i.e. measurement) is regulated by one or more regulatory bodies. The direct use of mA output CT for measuring load currents is not permitted in some jurisdiction as the only approved configurations call for the use of specified Ampere output CT. Where the use of Ampere output CT is dictated and a significant distance between the CT 110 and the measuring instrument 940 is desired, proposed solutions have comprised digitizing the secondary current of the CT proximate to the CT 110 and then transmitting the digitized signal via differential pairs, fiber optic cable, radio or other similar means to the distal measuring instrument 940. These solutions have proven unsatisfactory due to complexity, cost, and the need for a power source proximate the CT 110 in order to power the analog-to-digital converter.
What is needed is an apparatus and method for CT adaptation that provides for a measurement instrument to be located at a significant distance from a conductor carrying a load current to be measured.